Satellite based positioning system enabled receivers, for example Global Positioning System (GPS) receivers, are used widely for navigation and have substantial potential to provide location information in mobile wireless communication devices, including cellular telephones, which must soon comply with United States Federal Communications Commission E-911 location requirements.
One of the foremost interesting specifications of a GPS receiver is the time required to acquire satellite signals, known as spread spectrum pseudorandom noise (PN) codes, and to provide position coordinates after power is applied to the receiver. The time required to perform these operations is known as the time-to-first-fix (TTFF), which is determined generally by the hardware and software architecture of the receiver.
In battery powered hand-held GPS receivers, including those embedded in cellular telephones, the acquisition time influences total battery life since the receiver is powered continuously during location determination. The time required to generate a location fix is also important in emergency location applications, for example in E-911 enabled cellular telephones. Another important consideration, particularly in hand-held GPS receivers, is the time to acquire signals in weak signal environments, for example in environments where the signal is obstructed by foliage, automobiles, urban canyons and buildings.
It is known to search the code phase space for a single satellite in parallel. U.S. Pat. No. 6,009,118 to Tiemann, for example, discloses 2046 parallel correlators that search all phase delays for a single satellite. In “Real Time Missile Tracking”, Proceedings of the ION Aerospace Meeting”, April 1981, Wells describes a flash parallel correlator that computes correlation parameters for as many as 64 different phase delays of a single satellite. In these and other known parallel correlation schemes, a correlator block is assigned to search for one satellite over a pre-determined number of phase delays. In both Tiemann and Wells, however, the correlator block searched only for one satellite at a time. In these and other prior art systems, satellite signal searching remains a sequential process, wherein the parallel processing applies only to the phase delays for the particular satellite being searched. In Tiemann and Wells, searching or detecting more than one satellite in parallel requires duplication of the parallel correlators.
Other examples of parallel correlators are described in U.S. Pat. No. 5,901,171 to Kohli, and U.S. Pat. No. 6,208,291 to Krasner. In these patents a parallel correlator circuit is duplicated N times in order to search for N satellites.
In some applications, the satellite signal receiver has knowledge of the visible satellites, their approximate Doppler frequency, and in some cases, the approximate phase delay and phase/polarity of the 50 bits per second (BPS) navigation message bits. This knowledge can come from locally stored ephemeris, almanac, approximate position, and time, or from other sources, for example, the IS-801 specification provides satellite visibility, Doppler, phase delay at a particular epoch time. Since there are generally 8-10 satellites visible at any one time, searching in parallel for these signals shortens the total acquisition time.
What is desired generally is an efficient spread spectrum signal searcher that substantially reduces the average TTFF in a manner that minimizes the number of gates/transistors and in some applications reduces power consumption.
The various aspects, features and advantages of the disclosure will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description of the Invention with the accompanying drawings described below.